SynopsisIn this study bacterial luminescent toxicity tests (LTT) were used to monitor the residual toxicity concentration of a biocide-inhibitor after several months storage. The study was completed in two parts. The first study investigated the relationship between the chemical and toxicity concentrations of a biocide-inhibitor stored in mild steel containers and glass containers. After three months storage in the mild steel containers there was a significant difference (p < 0.01) between the chemically assayed level of the biocide-inhibitor and the toxicity level determined by the LTT. The results of the laboratory studies were confirmed in the second part of the study when the biocide-inhibitor was incorporated in the sea water used to flood a large undersea pipeline during the pressure testing programme. The disparity between the chemical and toxic assayed concentrations was not as great when the biocide-inhibitor was stored in glass containers. The influence ofthe container used for biocide testing plus the failure of the chemical analysis to reflect the toxicity level of this biocide-inhibitor demonstrates a different application of LTT. The results of this study also illustrate the usefulness and relevance of L' M' in testing the effectiveness of some biocides and indicate that LTT could be useful in monitoring the efficiency of biocides during their industrial application.